1. Field of the Invention
The present invention relates to an image reading device for reading image information as an electrical signal by relative scanning of photoelectric conversion means to an optically focused image.
2. Related Background Art
The image reading device for reading an image, obtained by focusing the light reflected from an original image or transmitted by an original film, with a line sensor such as CCD having a linear array of photosensor elements outputs, in the main scanning direction or the longitudinal direction of the line sensor, detected data corresponding to pixels from the photosensor elements in succession by electrical scanning, and the two-dimensional image information is obtained by mechanical scanning movement between the original image and the line sensor in the transversal or sub-scanning direction.
In order to handle the originals of plural sizes, there are a configuration having plural imaging lenses of different magnifications for matching the respective original sizes with the detecting line length of the line sensor, and switching the imaging lenses according to the original size, and a configuration having only one imaging lens of a magnification for matching the width of the largest-sized original with the detecting line length and, in case of reading a smaller original, projecting the original image with the above-mentioned magnification on a part of the scanning range of the CCD and outputting the data of such part only within the detecting line length.
FIG. 1 is a schematic view of the above-mentioned image reading device having plural lenses of different magnifications and designed for transmissive originals. An original stage 501, composed of a light transmitting flat plate, is horizontally fixed in a stationary position in the main body of the device. It is capable of supporting an original film F of various sizes, for example from a 35 mm film F1 (36.times.24 mm) to a 4.times.5 film F2 (4".times.5").
Above the original stage 501, there is provided an illuminating unit 502, which is so constructed as to uniformly illuminate the entire reading area of the original film F of the maximum size, for example the 4.times.5" film F2. In this case it is required to illuminate at least a circular area of a diameter equal to the diagonal length of the 4.times.5" film F2, so that there is required a large-sized light source 503 with a long illuminating optical path length L0.
A first imaging lens 505 can be positioned below the original stage 501, when a relatively small original such as the 35 mm film F1 is placed thereon, and focuses the light transmitted by the original F in the above-mentioned position onto the light-receiving face of a line sensor 508 positioned therebelow. The first imaging lens 505 has such an imaging magnification that the width for example of the 35 mm film F1 is matched to the detecting line length of the line sensor 508.
On the other hand, a second imaging lens 506 can be positioned below the original stage 501, when a relatively large original such as the 4.times.5" film F2 is placed thereon, and focuses the light transmitted by the original F in the above-mentioned position onto the light-receiving face of the line sensor 508 therebelow. The second imaging lens 506 has such an imaging magnification that the width of for example the 4.times.5" film F2 is matched to the detection line length of the line sensor 508.
The above-mentioned first and second imaging lenses 505, 506 are so supported as to be mutually switchable below the original stage 501 and are selected according to size of the original F, whereby the original film F of either size can be projected onto the light-receiving face of the line sensor below. The imaging lenses are switched manually by the operator according to his recognition of the size of the original film F.
The line sensor 508 is rendered capable of a scanning motion, in a plane parallel to the original stage 501, by linear movement mechanism which is not shown, and can read the image of the original film F of various sizes by the scanning of the projected image.
However, such image reading device which switches the plural imaging lenses according to the original size has been expensive because an imaging lens has to be provided for each original size. Also for maintaining the quality of the read image, in exchanging the plural imaging lenses for the originals of different sizes, an extremely high precision is required for the distance between the original and the photoelectric conversion means in each imaging lens and the perpendicularity of the optical axis thereof to the plane of the original. For this reason, the imaging system requires a positioning mechanism supported by a highly rigid structure and finished with an extremely high precision, and is therefore inevitably bulky and complex.
Also in the image reading device having only one imaging lens whose magnification is so selected as to match the width of the largest original with the detecting line length of the line sensor, in case of reading a smaller original, the detecting line length of the line sensor cannot be effectively utilized as the image of the original is projected only on a partial area of the detecting line, so that the resolution of the original cannot be made high.